I've personally experienced a RACP failure on my E46 M3 that had just plates welded on the underside. I made a thread documenting it including pictures of RACP replacement. The RACP began tearing away from the chassis given that it was the next direct weak point. Hence the need, as previously mentioned in the thread, for a more permanent fix like the Vince/CMP bar.

Question for those more knowledgeable on bushings, plates and xbrace aside. Isn't the idea of a bushing to allow some flex and to dictate where the flex occurs? If you just make everything super stiff and go solid bushings that offer no flex your just moving the weak point further down the line, i.e. the mounting points. So the idea of adding stiffer/solid bushings to any of the other fixes would just be adding more load to those areas? Is that basic assumption correct or is someone going to tell me I'm a moron who know nothing about how the world works?

Question for those more knowledgeable on bushings, plates and xbrace aside. Isn't the idea of a bushing to allow some flex and to dictate where the flex occurs? If you just make everything super stiff and go solid bushings that offer no flex your just moving the weak point further down the line, i.e. the mounting points. So the idea of adding stiffer/solid bushings to any of the other fixes would just be adding more load to those areas? Is that basic assumption correct or is someone going to tell me I'm a moron who know nothing about how the world works?

Stock bushings on plates is how it needs to be. The bushing concentrates the forces in a flexible media in a small footprint, while the plate on top of it absorbes it and spreads it out on a very large foot print.

Stock bushings on plates is how it needs to be. The bushing concentrates the forces in a flexible media in a small footprint, while the plate on top of it absorbes it and spreads it out on a very large foot print.

You've got all your suspension mounts (excluding RTA) and the sway bar mounted to the rear subframe. By isolating the subframe from the chassis with flexible bushings you're allowing all these pickup points to move as the subframe rolls and pitches beneath the chassis. This also occurs under throttle as we know due to the narrow subframe mounts the bushings (and chassis) flex quite significantly from the drive line torque.

By solid mounting the subframe your suspension points remain constant relative to the chassis making the car more stable and predictable especially at the limits of grip as this is when the bushing deflection is peaking. It also reduces body roll, wheel hop and in theory increase chassis rigidity in a localised area. Hard mounting the subframe turns two isolated rigid bodies into one thus the subframes rigidity is contributing to the chassis, bracing (like a strut brace) between the 4 subframe mounts. But only within.

However, as others have said, by eliminating the deflection and thus the dampening stock bushings would have provided your impulsive drive line forces are going to have a greater peak stress that's exerted on the chassis.

This is one of the reasons the RACP reinforcement kits I produce are quite substantial. The intention wasn't to stop stock cars cracking but allow for such modifications even if the power output had been increased significantly.

My personal experience with solid subframe bushings is that they should be a must for every E46 owner who likes to drive their cars properly. Negligible increase in NVH (everything mounted to the subframe is still isolated by their own rubber bushes) but the increase in feel and predictability and stability in the back end is phenomenal. Before that change (and some chassis bracing and monoball RTA bushes) I found the M3 to be fairly numb with both stock & poly bushes. Everyone who has driven my car has expected it to be unbearable but had been shocked by how compliant and comfortable it was and several converted to the same setup.

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